Frequency synchronizing apparatus



Jan.A 12, 1954 M. M. CARPENTER, JR 2,666,136

FREQUENCY SYNCI-IRONIZING APPARATUS Filed 08%.. 5l, 1 950 2 Sheets-Sheet l ryz- Jan. 12, 1954 M. M, CARPENTER, JR 2,666,136

FREQUENCY SYNCHRONIZ ING APPARATUS Filed oct. 51, 195o 2 Asheds-sheet 2 A j l l MM5/mz M. 2W/@95,76

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Patented Jan. 12, 1954 UNITED `STATES PATENT OFFICE ysigfnorto Radio Corporation of America, a corporation of Delaware Application October 31, 1950, Serial No. 193,182 4rClaims. (Cl. 250-27 .This invention relates to apparatus `forderiv- Ving .signals for synchronizing a local Yoscillatl'ir With'aburst'of alternating current energy.

in particular, this invention relates to apparatus for deriving afrequency or phase control sig- .nal :from 4a Vburst of control frequency transmitted in acolor television system :of thetype wherein the colorinformation ismultiplexed at an elemental rate.

Inra dot ymultiplex color televisioni system succussive elements of the scannedlineof the raster are represented in accordance with an intensity of a ,diierent'component color. The first element mightcorrespond to the red intensity ,of -that pointof the image, a second element might correspond togreen, and a third element might correspond to blue.. Thissequence may then'be repeated during the rest ofthe line. In some forms of sucha system `thesecond scanning of the Vsame line position iszdone in .such manner that the information as to the red, green and `bluecontentof theimageis doubly interlaced, which means thatit is shifted along the line with 'respect .to theline by `180 degrees. This means that` ared element will now appearhalf way be- 'tweenthe red elements produced when the line vwas `scanned initially.

` Itis apparentthat somemeansmust be provided for synchronizing the color reproducing appar'atus at the receiver with the transmitted signals, so that the colors reproduced `attlieA receiver will correspond :to the signals transmitted.. In other words if the elemental `information being received ata particularftime is indicative ofc-the intensity of the red .componentin thelimagewat a particular point, the reproducing mechanism1 in 'the receiver mustbenoperatedso as to` reproduce redatra ccrrespondingpoint in the A.reproduced image.

',Heretofora a free-running sampling oscillator .ihasrbeen employed'tofkey the color reproducing means inthe receiver `at the appropriate intervals. Thefrequencyfof this oscillator` has been controlled by comparing` its output with a burst oflalternating currentgenergy occurringimmedii ,atelyafter the `l'iorizonalsync pulses. This burst oflenergy has Ythe same `phase and frequency as `the change in color information conveyed by the transmitter.` `In order thatl thesampling oscillator be controlledbythe burstnandr notby any other energyoccurring at the transmittedwave, itis apparent that means mustbe provided for .separating the burst` from the ,rest of `the trans- .;mitted information.v VAsth-e frequency of the 'burst is precisely established, ltcanwbe `selected ai by `'appropriate lters. Generally, the burst `is superimposed uponthe blanking pulse so that it occurs at the blacker than black region. Accordingly, amplitude selection can also `besemL ployed..` `Perhaps the most reliable Way of sep arating this burst, however, is to apply it to a gating circuit that is adapted to pass signals Pap*- plied to it only during the time thatv the burst is present. After the burst or" synchronizing s energy has been separated by one or more conibinations of the above methods, it has been customary to apply it to a` phase discriminatorto which the output ofthe sampling oscillator is also supplied. The directcurrent control voltage developed bythe phase discriminator is then applied to a reactancetube that is associated with the sampling oscillator in such a way `as to control its frequency.

In accordance with this invention,` however, the function of separating the lburst from the rest or" the transmittedtrain-of signalsand of `de riving a directA current voltage that is dependent upon the phase relationship `between this burst of energy and the output of the sampling oscillator is accomplished by a single circuit in a very simple manner.

It isrtherefore, an object of this inventionto provide an improved and simplified means for deriving a, frequency controlfvoltagein response to the phase relationship between alburst of ali ternating current lenergy appearing-during preA` determined intervals in the transmitted wave and theoutput of the sampling oscillator that is used to control the multiplexing at a color television receiver.

Briefly, this objective maybe obtained by combining a knownphase discriminator with apparatus for rendering said` phase discriminator operative` during the time when a burstof` synchronizing alternating currentpenergy is .being` received.

The mannerin which the` above objective may bef achieved and the advantages'cf this invention realized will become more apparent from adetailed consideration of the drawings in which: Figure l'illustrates one Way of applying the principles of this invention to a phasediscriminator employing unilateral conducting devices in a bridge network.

Figure 2` illustrates voltage Waves that will help in the explanation of the circuit shown iii-Figure l.

Figure 3 also illustratesvoltage waves helpful inthe explanation of .the operation of the cir cuit shownin Figure 1. Y

The` circuit yarrangement of Figure 4 illustrates another way of applying the keying potentials to a phase discriminator of the type shown in Figure 1, and

Figure 5 illustrates a method of applying this invention to a phase discriminator that operates in a manner similar to Figure l which employes fewer unilateral devices.

Turning now to Figure 1, there is shown a source 2 of video waves such as illustrated in waveform 4 of Figure 2. The pulse 6 of Figure 2 is the well known horizontal synchronizing pulse and the burst of alternating current energy 8 occurs immediately following the pulse 6 at a portion of the video signal train known as the back porch. The top of the blanking pulse 'I is indicated by the numeral I and a portion of one line of video signals is indicated by the numeral I2. As previously stated, the amplitude of the transmitted voltage during the time I2 successively represents each of the plurality of component colors employed in the color television system. For example, during the interval R the intensity of the red component occurring at that point in the image is transmitted. During a succeeding interval G the green information at that point in the image is transmitted, and during a third portion B the information as to the intensity of the blue at that point in the image is transmitted. Such a signal may be derived from the second detector or at the output of the Video amplifiers of a standard monochrome receiver such as are now available.

The video signals supplied by the source 2 are coupled into series relationship with a storage condenser I across which the control voltage is to be developed. This control voltage is applied to control a reactance tube II which in turn controls the frequency of a sampling oscillator I Qin a manner well known to those skilled in the art.

This series circuit is connected between a terminal I4 and a terminal I6 of a rectifier bridge network I8. A similar bridge network is described in the United States Patent 2,250,284 issued in July 1941 to Wendt. For the sake of convenience, however, this bridge network I8 will now be described in detail. The upper arms of the bridge are comprised of unilaterally conducting devices such as diodes 26 and 22 lconnected in series opposition and having their cathodes connected to the terminals I4 and I6 respectively. Their plates are tied to another terminal of the bridge indicated by the numeral 24. The lower two arms of the bridge network I8 are comprised of unilateral conducting devices such as the diodes 26 and 28 that are connected in series op position between the terminals I4 and I6 respectively. The cathodes are connected to a terminal indicated by the numeral 30.

Between terminals 24 and 30, or in other words across one of the diagonals of the bridge network I 8, the following apparatus is connected in series. First of all there is a secondary 36 of the transformer 38 having a primary 4U coupled to the output of the sampling oscillator I9. In series with the secondary 38 there is connected an electronic switch 42 vwhich may take any desirable form.

This particular form of electronic switch operates in the similar manner to the keyed clamp circuits such as described in United States'Patent 2,299,945 to Karl Wendt. It is comprised of a first diode cr other unilateral current conductingdevice 44 having its cathode connected to the lower end of the secondary 36 and its plate coupled via a blocking condenser 46 to a means for providing positive pulses 41 during the burst in- Y terval. A second unilateral conducting device, which may be a diode 48, has its plate connected to the bottom of the secondary 36 and its cathode coupled to means for providing a negative pulse 49 during the burst interval. Between the plate of the diode 44 and the cathode of the diode 48 is connected a potentiometer comprised of a resistor 52 and amovable arm 54. Between the movable arm 54 and the terminal 4U of the bridge network I8, there is connected an energy storage means 5'! which may be comprised of a resistor 56 connected in parallel with a condenser 58. The particular order in which the secondary 38, the

. electron switch 42, and the energy storage circuit 5'I are connected is not of importance to the practice of this invention.

The overall operation of the apparatus just described in connection with Figures l, 2 and 3 will now be explained. It is well understood by those skilled in the art that voltages appearing across one diagonal of a balanced bridge network such as I8 of Figure 1 do not appear across the opposite diagonal and vice versa. Therefore, in the arrangement shown, the voltages appearing between the terminals 24 and 30 do not appear at the terminals I4 and I6 and, therefore, do not effect in and of themselves the charge appearing across the storage condenser I5. However, the voltage appearing at the terminal I4 will appear at the terminal I6 and thus across the storage condenser I5, when the diodes are conducting.

However, in order for a voltage applied at the terminal I4 to reach the terminal I6, the diodes 20, 22, 26 and 28 must be in a conducting condition. The conductivity of the diodes is controlled by the voltages appearing in the series circuit connected between the terminals 24 and 3G. Upon application of the positive pulse 41 and the negative pulse 46 to the electronic switch 42 the circuit between the terminals 24 and 38 of the bridge network I8 is completed. During this time the voltage having the frequency of the oscillator I9 is inserted between the terminals 24 and 36 by the action of the transformer 38. One cycle of this voltage is indicated by the waveform 62 of Figure 3. During the positive portion 'ofi the waveform 62 sufficient circulatory current is drawn through the diodes to charge the'condenser 58 to a level indicated by the dotted line 63. This, vin effect, biases the diodes into a nonconducting condition for a greater portion ofthe cycle of the sampling wave provided by the oscillator I9. The conducting portion of the cycle, therefore, lies within the time interval defined by the dotted lines 64 and 65 of Figure 3. f When the diodes are conducting, it is possible for the burst of alternating current energy then present at the terminal I4 to appear at the terminal I6. -One cycle of this waveform is indicated'by the numeral 66 in Figure'S. During the positive portion of the waveform 66 that occurs during the conducting period of the diodes, that is during the nearly triangular portion indicated by the numeral 68, the condenser I5 is charged so that its top plate becomes positive with respect to ground. The charge path starts at ground, goes through the source of video signal 2, the diode 26, the terminals 30 and 24, the diode 22, the condenser I5 itself and back to ground. During the portion of the negative swings 69 ofthe waveform 66, the condenser I5 is charged inthe opposite direction. Therefore, if the waveform 66' passes throughV a ground potential or reference potential midway through the conducting period 5 idenedf by the dotted-lines?Jl Mandi 65=v ofFigure 13, the condenser l 5- is chargediby a f-given amount and-thenf discharged byi-the-vsame amount. I 'The net charge therefore" on `the condenser i 5 isser@ and 'the reactance tube fiL-andhence thefrc quency f of `the.` oscillator 9,- remains unchanged. HHowever,`l if 1 the' phase relationship; between theV burst: of` frequency` as represented by. the curve 6 and; the. oscillator frequency :fasi` repre- 'sented by the waveformfz: should.:change,vthen either aipositive orznegativevoltage is :built across ithestorage condenser` I5: and thereactancetube rchanges'thefrequency of: the oscillator :I9: as required.

.fAndmportant feature of .this invention` isV the 4fact 'that the application ofthe keying.` pulses "4l and `49 scrveto makefthesbridge' `network IB operative during the `burstinterval only..

Referring to. `Figure 1i` there is` shown a.. slightly ldiierent arrangement.; ior'- applying.. .the 'prim replaced by aseries resistor 19. Negative pulses 41are applied by a coupling condenser to the lower end (of the resistor. l'il and .positive pulses 49 are applied via the coupling condenser "46' to the upper end ofthe resistor it.

If the sum of the amplitudesof the pulses 'si and 49 is greater than the maximum possible diiference betweennthe video signal supplied by the source 2 and the sampling oscillator voltage wave introduced `by the secondary 35, `the bridge circuit Aoperates as before. The reason for this .requirement` .in the relative amplitudes. I of. the `pulses 4'! and 49 is thatothesewpulses largely determine the amount ofucharge onltheondenser 58. If the charge in `the condenser .59 is insufiicient, the maximum swings of the video signals produced as a line is 'being scanned may be combined with the positive portion of the oscillator frequency appearing in the secondary 36 so as to provide a charge path for the condenser IE. Under certain circumstances this would produce an erroneous control voltage across the condenser i5, as it would be subject to the video signals derived from the image and appearing at the terminal IS. However, by increasing the amplitude of the pulses il and 49 the bias on the diodes can be made great enough so as to prevent the condenser i5 from being charged or discharged at any time except when these pulses occur or at any time except the burst interval.

The arrangement shown in Figure 5 illustrates another Way in which the principles of this invention may be incorporated into a phase discriminator employing only 2 diodes. Such a phase discriminator will be found in Wendt Patent 2,250,284 noted above. A source of video signal 'i2 that is similar to the source 2 described in connection with Figures l and 4 and a storage condenser 'i4 that is to perform the same funcM tion as the storage condenser i5 of Figures 1 and 4 are connected in series between terminals 'iS and 18. The voltage across the storage condenser 'M is employed, as is known to those skilled in the art, to control the conductivity of the reactance tube B9 which, in turn, controls the frequencyf'of a sampling oscillator?Av E2."^'UBetween the terminals `l6 andi-"IBFarefconnected charge and discharge paths for the condenser-T114. "-The charge path is comprisedof aunilateralconducting'fde'vice'suchf as aA diode/84, an energyf storage circuit "di which may becomprisedof a con# denserand resistor connectedin parallel;4 and a portionrl` of a secondaryvoi artransformerili! having its primary. energizedby ther sampling frequency voltage Wave'suppli'ed byitheioscillator t2. Between thelower end ofthefsecondaryl and the terminal "Hiv` there is connectediia resistor 190.

The discharge path is comprised of aunilateral conducting :dei/ice, `which mayibe a` .diodei9l, connected in series :withan energystoragencirlcuit 92, which y may' be comprisedl of a i condenser and "resistor connected `in iparallel. .The upper endfof the storage. circuit 92 isconnectedyia a portion 93 ofthesecondary oftheatransformer 89. Between .theupper 4end of .thisfpcrtiona of :the secondary 4and .the terminal lt'iiwiscoilnected-ea resistor `94. Thediodesfl'randfzi are'fdissimilarly connected soA that :the cathodesofvthe diode` .84. is .crmnectednto the terminal.' 'it and-.the plate of the diode 9i is connected to the terminal l.. .lositive pulses` occurring. :during the burst interval :are coupledwtonthe;upper .end4 of `the resistor tu via thexcondenserr; .andi similarly negative pulsesare coupled `to theilowcrwendiof the resistor 9d via a condenser 96.

The operation of the circuit. arrangement shown in Figure 5 isasfollows. Upon theoccurrencenof positive' and negative pulses91 and99 acrossithe series'resistcrs and eil, thevdiodes :4 and ci conduct during a portionpf eachcyclefof the voltage vwave supplied by the foscillator. 82 4so that theupperplates of the condensers inthe energy storage circuits `-8E and 532 are charged.` negatively. Thislplaces a bias on `theidiodes-M `and y9| `and prevents. their conductance except during these pulseintervals when the is overcome. i Iffthe burstfreouency of,` synchronizing wave is-similar to that illustrated bythe curve 66 shown by Figure 3, the condenser "le charges in a positive direction during the positive portions of the burst and discharges in the negative direction during the negative portions of the burst. Thus, as in connection with Figures 1 and 4, the diodes 84 and 9i become conductive only during a small portion of a cycle of this frequency. During this conduction period, the burst of synchronous frequency that is applied by the source l2 to the terminal 16 is permitted to charge and discharge the storage condenser 14. Then, as was explained in connection with Figure 3, the phase relationship between this latter burst frequency and the period of conductivity or the phase relationship between this burst frequency and the frequency supplied by the oscillator 82 determines whether the charge condenser 14 is charged negatively or positively.

Having thus described my invention, what is claimed is:

1. Apparatus adapted to provide a frequency control voit-age in response to the phase difference between a burst of synchronizing voltage and the output of an oscillator comprising in combination a rst pair of diodes connected in series parallel with a second pair of diodes of like polarity, an energy storage circuit, the secondary winding of a transformer, an electronic switch adapted to supply pulses of such polarity as to urge said diode into a conducting condition, said energy storage circuit, said secondary winding and said switch being connected in series parallel relationship with said pairs of diodes, and said switch being adapted to operate only when said burst is present.

2. Apparatus as described in claim 1 in which a source of Video signals is connected in series with a condenser across which the control voltage is to be developed, said series combination being connected between the junction between said iirst pair of diodes and the junction between said second pair of diodes.

3. Apparatus adapted to produce a frequency control voltage in response to the phase relationship between a burst of synchronizing alternating current and the output of an oscillator comprising the combination of a diode connected in series with a parallel combination of a resistor and a condenser, and in series with a portion of Y a transformer secondary, and in series with a resistor, a second diode having its plate tied to the cathode of the first diode, a parallel combination of a resistor and a condenser, another portion of said transformer secondary, and a resistor connected between the cathode of said second diode and said rst mentioned resistor, a condenser and a source of video signals including said burst conage waves, one of which occurs in bursts, the

other being continuous comprising in combination, rst and second terminals, a source of signals in which said bursts occur at predetermined intervals, a storage condenser coupled in series with said source, said series combination being 'L connected between said terminals, a first unilateral current conducting device coupled between said terminals, a second unilateral current conducting device coupled between said terminals and in parallel with said iirst unilateral device, said first and second unilateral devices being connected so as to pass current in opposite directions between said terminals, means for biasing each of said unilateral current conducting devices to cut off, means for applying said continuous alternating waves in series with eachv of said unilateral devices, the amplitude of said waves being less than the cut-off bias applied to said unilateral current conducting devices, means for applying keying pulses in series with each of said unilateral devices, said keying pulses being of such polarity as to tend to overcome said cut-off bias applied to each of said unilateral current conduction devices, the amplitude of said keying pulses being such that when combined with the amplitude of said continuous wave, the cut-oi bias applied to said unilateral current conducting devices is overcome for a portion of each cycle of said continuous alternating current wave occurring during said keying pulses thus permitting current to flow through one of said unilateral devices and charge said storage condenser in one polarity during a portion of the positive excursion of the alternating current wave occurring in bursts and to flow through the other unilateral current conducting device and charge said storage condenser in the opposite polarity during a portion of the negative excursions of the alternating current wave occurring in bursts so that the net charge on said condense;` is dependent on the phase relationship between the alternating current waves occurring in bursts and the continuous alternating current waves.

MARSHALL M. CARPENTER, JR.

References Cited Vin the file of this patent UNITED STATES PATENTS Number Name Date 2,358,545 Wendt Sept. 19, 1944 2,532,338 Schlesinger Dec. 5, 1950 2,559,719 Hershberger July 10, 1951 2,588,094 Eaton Mar. 4, 1952 quam. li. 

